The incidence of fungal infections has increased substantially over the past two decades and invasive forms are leading causes of morbidity and mortality, especially amongst immunocompromised or immunosuppressed patients. Disseminated candidiasis, pulmonary aspergillosis, and emerging opportunistic fungi are the most common agents producing these serious mycoses. It is a particular feature of fungi that they are able to generate an extracellular matrix (ECM) that binds them together and allows them to adhere to their in vitro or in vivo substrates. These biofilms serve to protect them against the hostile environments of the host immune system and to resist antimicrobial killing (Kaur and Singh, 2013).
Pulmonary aspergillosis can be segmented into those patients suffering with non-invasive disease versus those with an invasive condition. A further sub-division is used to characterise patients who exhibit an allergic component to aspergillosis (known as ABPA; allergic bronchopulmonary aspergillosis) compared with those that do not. The factors precipitating pulmonary aspergillosis may be acute, such as exposure to high doses of immuno-suppressive medicines or to intubation in an intensive care unit. Alternatively, they may be chronic, such as a previous tuberculosis (TB) infection (Denning et al., 2011a). Chronic lung infections with Aspergillus can leave patients with extensive and permanent lung damage, requiring lifetime treatment with oral azole drugs (Limper et al., 2011).
A growing body of research suggests that Aspergillus infection may play an important role in clinical asthma (Chishimba et al., 2012; Pasqualotto et al., 2009). Furthermore, recently published work has correlated Aspergillus infection with poorer clinical outcomes in patients with chronic obstructive pulmonary disease (COPD) (Bafadhel et al., 2013). Similarly cross-sectional studies have shown associations between the presence of Aspergillus spp. and Candida spp. in the sputum and worsened lung function (Chotirmall et al., 2010; Agbetile et al., 2012).
Invasive aspergillosis (IA) exhibits high mortality rates in immunocompromised patients, for example, those undergoing allogenic stem cell transplantation or solid organ transplants (such as lung transplants). The first case of IA reported in an immunocompromised patient occurred in 1953. This event was concurrent with the introduction of corticosteroids and cytotoxic chemotherapy into treatment regimens (Rankin, 1953). Invasive aspergillosis is a major concern in the treatment of leukaemia and other haematological malignancies given its high incidence and associated mortality. Death rates usually exceed 50% (Lin et al., 2001) and long term rates can reach 90% in allogeneic hematopoietic stem cell transplantation recipients, despite the availability of oral triazole medicines (Salmeron et al., 2012). In patients undergoing solid organ transplantation (particularly of the lung), the use of high doses of steroids leaves patients vulnerable to infection (Thompson and Patterson, 2008) which is a serious problem. The disease has also appeared in less severely immunocompromised patient populations. These include those suffering with underlying COPD or cirrhosis, patients receiving high dose steroids, and individuals fitted with central venous catheters or supported by mechanical ventilation (Dimopoulos et al., 2012).
Existing anti-fungal medicines are predominantly dosed either orally or systemically. These commonly exploited routes of delivery are poor for treating lung airways infections, since drug concentrations achieved at the site of infection tend to be lower than those in organs. This is especially so for the liver, which is a site of toxicity: up to 15% of patients treated with voriconazole suffer raised transaminase levels (Levin et al., 2007; Lat and Thompson, 2011). Exposure of the liver also results in significant drug interactions arising from the inhibition of hepatic P450 enzymes (Jeong, et al., 2009; Wexler et al., 2004).
Furthermore, the widespread use of triazoles, both in the clinic and in agriculture has led to a growing and problematic emergence of resistant mycoses in some locations (Denning et al., 2011b; Bowyer and Denning, 2014).
It is clearly evident that an urgent medical need exists for novel anti-fungal medicines that deliver improved efficacy and better systemic tolerability profiles.